1. Field of Invention
This invention relates to a method of coating an automobile body.
2. Description of the prior art
Conventionally, the coating of an automobile body is carried out by four processes; namely, a pretreating process for degreasing, an undercoating electrodeposition coating process for rust-proofing, an additional corrosion-resistance coating process and a final finish coating process for imparting a desirable finish to the treated body.
To produce an automobile which is light in weight, and having superior rust-proofing properties, it has been proposed in recent years to produce a portion of the automobile body with primarily plastic parts, having a lesser amount of sheet metal parts for the remaining parts. In this case, it is required to coat both the plastic parts, as well as the sheet metal parts.
In general, plastic parts useful for automobile body parts have a surface resistivity of around 10.sup.10 -10.sup.20 ohm-cm, which is markedly higher than that of sheet metal parts. It is therefore difficult to provide plastic parts with an electrodeposition coating or an electrostatic coating by using the same process conditions which are used for sheet metal parts.
In view of the above difficulty, several techniques have been suggested. For example, an electrically conductive film (metallic film) can be formed at the surface of a plastic product before coating by non-electrolytic plating and the like (Japanese Patent Application Publication Gazette No. 61-38936). Alternatively, an electrically conductive film can be formed by coating a conductive primer on a plastic product as a coating foundation so as to make electrostatic coating possible. (Japanese Patent Application Laying Open Gazette No. 58-143867).
However, even if these procedures (formation of electrically conductive film) are used on plastic parts of an automobile body as an undercoating, the sheet metal parts must also be undercoated by electrodeposition coating. This means that sheet metal parts and plastic parts are separately given an undercoating treatment, after which both types of parts are assembled for a second coating and a finish coating. The assembling process interposed between the undercoating process and the second coating process is problematical in that the coating film provided on sheet metal parts by the undercoating process may scratch, resulting in a deterioration of the body part. The scratching of the coating film can be reduced or prevented by performing the assembly work more carefully, but the assembly time is unduly prolonged.
Plastic parts are usually lower in heat-resistance than sheet metal parts. Therefore, it is difficult to produce coating films on both plastic parts and sheet metal parts under the same conditions. For example, plastic parts made of PC (polycarbonate) resin or ABS (acrylonitrile butadiene styrene copolymer) resin must be dried (for baking of coating film) at a low temperature (about 90.degree. C., for example) for many hours.
On the other hand, if plastic parts are made of resin materials containing reinforcing fibers, such as glass fiber, heat-resistance can be improved but the smoothness of the surface of plastic parts is worsened by the presence of the reinforcing fibers. The smoothness of the surface is not improved even by the second coating and the finish coating processes.
In the case where plastic parts and sheet metal parts are separately given an undercoating as mentioned above, differences in color tone of the coating films between the two types of materials will remain after the finish coating. Thus, it is difficult to match the color of both types of parts.